The present invention relates to the field of electromechanics and more particularly to the field of microelectromechanical devices.
Thin film processes developed in the field of microelectronic integrated circuits have been used to produce precision microelectromechanical devices. For example, solid state laser and fiber optic couplings, ink jet nozzles and charge plates, magnetic disk read/write heads, and optical recording heads have been produced using thin film processes including photolithography, sputter deposition, etching, and plasma processing. These thin film processes allow the production of microelectromechanical devices with submicron dimensional control.
One important microelectromechanical device is an electrostatically driven rotating mirror which is used in an optical scanner such as a bar code reader. In particular, an electrostatically driven torsional scanning mirror is discussed in the reference entitled xe2x80x9cSilicon Torsional Scanning Mirrorxe2x80x9d by Kurt E. Petersen, IBM J.Res.Develop., Vol. 24, No. 5, September 1980. In this reference, a single-crystal silicon chip contains a mirror element attached to two single-crystal silicon torsion bars. This silicon chip is bonded to another substrate into which a shallow rectangular well has been etched. At the bottom of the well, two electrodes are alternately energized to deflect the mirror element in a torsional movement about the silicon torsion bars.
The silicon torsion bars, however, may be unnecessarily stiff thus requiring excessive torque to rotate the mirror. In addition, the location of the electrodes in the path of the rotating mirror may restrict the rotation of the mirror. Increasing the distance between the electrodes and the mirror may reduce the electrostatic force generated therebetween. Furthermore, the bonding of the silicon chip to the second substrate may add unnecessary complication to the fabrication of the device.
A two-dimensional optical scanner is discussed in the reference entitled xe2x80x9c2-Dimensional Optical Scanner Applying a Torsional Resonator With 2 Degrees of Freedomxe2x80x9d by Yoshinori Ohtuka et al., Proceedings, IEEE Micro Electro Mechanical Systems, 1995, pp. 418, 306-309. This reference discusses a torsional vibration system where two vibration forces are produced by one driving circuit. In particular, bimorph cells are used to excite the torsional vibration. One-dimensional scanning is enabled by driving the bimorph cells with the resonance frequency of either of the two torsional vibrations. Two-dimensional scanning can be achieved if the bimorph cells are operated by adding the resonance frequency signals of the two torsional vibrations. The scanner of this reference, however, may only be able to independently scan in any one dimension at predetermined resonance frequencies. In other words, because a single driving circuit is used to excite vibration about two axes, vibration about either axis may be limited to predetermined resonance frequencies. The scanner of this reference may also require the assembly of discrete components.
Notwithstanding the above mentioned references, there continues to exist a need in the art for improved microelectromechanical scanners and methods.
It is therefore an object of the present invention to provide improved electromechanical devices and methods.
It is another object of the present invention to provide an electromechanical rotating plate including improved actuators.
It is still another object of the present invention to provide an electromechanical rotating plate which can reduce the torque needed to effect rotation.
It is still another object of the present invention to provide an electromechanical rotating plate which can independently rotate around two different axes.
These and other objects are provided according to the present invention by electromechanical devices including a frame having an aperture therein and a plate suspended in the aperture. A pair of beams extend from opposite sides of the plate to the frame wherein a first end of each of the beams is fixedly connected to one of the plate and the frame and the second end of each of the beams is in rotational contact with the other of the plate and the frame so that the plate rotates relative to the second frame about an axis defined by the beams. Accordingly, the plate is free to rotate about the axis thus requiring relatively little torque to effect rotation.
Furthermore, the electromechanical devices can include an actuator having an electrode spaced apart from the frame and an arm extending from the electrode to a portion of the plate so that a potential difference between the electrode and the frame results in an electrostatic force which is transmitted by the arm to the plate thus effecting rotation of the plate. Because this actuator generates an electrostatic force in response to a potential difference between itself and the frame instead of the plate, the actuator does not inhibit motion of the plate. In addition, this actuator can provide a biasing support for the plate.
According to one aspect of the present invention, an electromechanical device includes a first frame having a first aperture therein, a second frame suspended in the first aperture wherein the second frame has a second aperture therein, and a plate suspended in the second aperture. A first pair of beams support the second frame along a first axis so that the second frame rotates about the first axis. A second pair of beams support the plate along a second axis so that the plate rotates about the second axis. The first axis and the second axis preferably intersect at a 90xc2x0 angle providing independent rotation for the plate about both axes. A first actuator provides mechanical force for rotating the second frame relative to the first frame about the first axis. A second actuator provides mechanical force for rotating the plate relative to the second frame about the second axis. Accordingly, the plate can be independently rotated relative to the first and second axes.
The first and second frames can be formed from a microelectronic substrate to provide a microelectromechanical actuator. The plate can also be formed from this microelectronic substrate. Accordingly, the two axis actuator can be fabricated on a single substrate without the need for wafer bonding. More particularly, the first and second frames and the plate can be formed from a silicon substrate and the beams can be formed from polysilicon. The microelectromechanical actuator can thus be fabricated using thin film processing techniques known in the field of micromachining.
Each of the beams supporting the plate can extend from an opposite side of the plate to the second frame, and a first end of each of the beams can be fixedly connected to one of the plate or the second frame. The second end of each of the beams can be in rotational contact with the other of the plate or the second frame so that the plate rotates relative to the second frame about the axis defined by the beams. More particularly, these beams can be fixedly connected to the plate, and each beam may include an arched contact surface adjacent the second frame so that each of the beams rolls on the second frame as the plate rotates. The arched contact surfaces further reduce the torque required to rotate the plate.
A biasing support can support the plate relative to the second frame so that the plate and the second frame are coplanar when no mechanical force is provided by the second actuator and so that the plate rotates about the second axis when mechanical force is provided by the second actuator. This biasing support can be provided by the actuator. In particular, the second actuator can include an electrode spaced apart from the second frame and an arm extending from the electrode to a portion of the plate wherein a potential difference between the electrode and the second frame results in electrostatic force which is transmitted via the arm to the plate thus rotating the plate relative to the second frame. The electrode can be fixedly connected to the second frame along a portion thereof spaced from the plate, and the arm can be fixedly connected to the plate so that the plate and the second frame are maintained in a common plane when there is no potential difference between the electrode and the second frame. Alternately, a micromechanical spring can be provided between the plate and the second frame.
An insulating layer can be provided between the second frame and the electrode of the second actuator to prevent electrical shorts therebetween. For example, a silicon nitride layer can be provided on the second frame. In addition, the arm of the actuator may extend to a portion of the plate closely spaced from the second axis. Accordingly, a relatively small movement of the actuator can result in a relatively large rotation of the plate about the second axis.
According to another aspect of the present invention, a method for fabricating an electromechanical device on a substrate includes the steps of defining plate and frame regions on a face of the substrate wherein the frame region surrounds the plate region and wherein the plate region and the frame region are separated by a sacrificial substrate region. A supporting structure is formed to support the plate region along an axis relative to the frame region, and an actuator is formed on the face of the substrate which provides mechanical force to the plate region. The sacrificial substrate region is then removed so that the plate region rotates about the axis relative to the frame region in response to mechanical force provided by the actuator. This method allows the fabrication of a microelectromechanical device with a rotating plate using a single substrate thus eliminating the need for wafer bonding.
More particularly, the steps of defining the plate and frame regions may include doping the respective regions, and the step of removing the sacrificial substrate region may include etching undoped portions of the substrate. Accordingly, the plate and frame regions can be defined early in the fabrication process and then separated later in the fabrication process after forming the beams and the actuators. Accordingly, the plate and frame regions can be defined without creating significant topography allowing the beams and actuators to be formed on a relatively flat substrate.
The step of forming the supporting structure can include the steps of forming a pair of beams on opposite sides of the plate region which define an axis of rotation through the plate region. Each of the beams extends from the plate region to the frame region, and each of the beams is fixedly connected to one of the plate region and the frame region. A second end of each of the beams is in rotational contact with the other of the plate region and the frame region so that the plate rotates relative to the frame. As discussed above, the rotational contact reduces the torque required to rotate the plate.
The step of forming the beams can include the steps of forming a sacrificial layer on the substrate, forming first and second holes in the sacrificial layer exposing portions of the plate region along the axis, and forming first and second partial holes in the sacrificial layer opposite the frame region without exposing the frame region. The partial holes are formed along the axis, and the partial holes can be formed by isotropically etching the sacrificial layer. First and second beams are formed on the sacrificial layer wherein each of the beams is fixedly connected to the plate region through a respective one of the holes in the sacrificial layer. Each beam extends from a respective exposed portion of the plate region to a respective partial hole opposite the frame region. The sacrificial layer is then removed so that the first and second beams extend from the plate to the frame in a cantilevered fashion. Accordingly, each of the beams includes an arched contact surface in rotational contact with the frame.
The step of forming the sacrificial layer may include the stops of forming a first sacrificial sublayer having a first etch rate and forming a second sacrificial sublayer having a second etch rate which is high relative to the first etch rate. The step of isotropically etching the sacrificial layer thus forms the partial holes primarily in the second sacrificial sublayer. The first sacrificial sublayer with the relatively low etching rate thus ensures an adequate spacing between the contact surface of the beam and the substrate.
The step of forming the actuator can include the steps of forming an electrode spaced apart from the frame region and an arm extending from the electrode to a portion of the plate region. A potential difference between the electrode and the frame region results in electrostatic force which is transmitted by the arm to the plate region. Accordingly, the plate can rotate in response to the electrostatic force generated between the electrode and the frame. Furthermore, by providing a fixed connection between the arm and the plate, the actuator can provide a biasing support which supports the plate relative to the frame so that the plate and the frame are coplanar when no mechanical force is provided to the plate.
Electromechanical devices of the present invention can thus provide independent rotation of the plate about two axes of rotation. The beams which provide a rotational contact between the plate and the frame can reduce the torque required to rotate the plate. Furthermore, the electrostatic actuators which generate a mechanical force in response to a potential difference between the electrode and the frame need not lie in the path of rotation of the plate. Electromechanical devices of the present invention can also be fabricated on a single substrate using micromachining techniques.
By providing a reflecting surface on the plate, a rotating mirror for a scanner can be produced. Accordingly, a rotating mirror can be produced efficiently and economically without the need for wafer bonding or the assembly of discrete components.